1. Field of the Invention
This invention relates to a sheet feeding apparatus for feeding sheets (including original documents) in image processing systems such as a printer, a copying machine, a stencil duplicating machine, and a document reader.
2. Description of the Prior Art
FIG. 8 of the accompanying drawings shows a sheet feeding apparatus for a printer. In operation, a sheet feed roller 202 (called "the feed roller 202") successively pays out sheets 201 from a sheet tray 200 from top to bottom. Each paid out sheet 201 is separated from the remaining sheets 201 by a separating roller 203 and a separating pad 204, and is fed into a space between a pair of register rollers 205. The register rollers 205 are rotated in synchronization with the rotation of a printing drum 206, so that an image perforated on a stencil wrapped around the printing drum 206 is transferred (or printed) onto the sheet 201. Thereafter, the sheet 201 is conveyed to a discharge tray 208 via an absorbing unit 207.
The feed roller 202 has at least a surface made of a material with a high friction coefficient such as rubber. A frictional force between the feed roller 202 and a top sheet 201 is designed to be larger than a frictional force between the sheets 201. The former frictional force depends upon a sheet feeding pressure (i.e. a pressure applied by the feed roller 202 to the sheets 201). The smaller the sheet feeding pressure, the oftener the feed roller 202 slips on the sheet 201, and fails to pay it out. Conversely, the larger the sheet feeding pressure, the oftener the feed roller 202 feeds a plurality of sheets 201 at a time.
Therefore, it is necessary to maintain the sheet feeding pressure constant, since it is continuously variable with factors such as a kind, a size and an amount of sheets 201, a printing speed, and so on. Referring to FIG. 8, a sheet feeding pressure regulator 209 (called the "feeding pressure regulator 209") regulates the sheet feeding pressure, and includes an arm 211 which is connected at its base to a stationary shaft 210, has a weight 212 at its free end, and is longitudinally movable at the free end. The arm 211 supports the feed roller 202 at the free end, so that the feed roller 202 is freely rotatable. The feeding pressure regulator 209 urges the arm 211 upward via a spring 214 connected to a side plate of the printer via a regulating member 213. The regulating member 213 includes a plurality of hooks, with which the spring 214 is engaged so as to regulate the sheet feeding pressure.
A pressure by which the sheets 201 comes into contacts with the separating roller 203 affects sheet separating functions. A separating pressure regulator 215 is provided in order to regulate this pressure, and comprises a spring 216 which is engaged with the separating pad 204 at its one end, and with a member 217 at the other end thereof. The member 217 or the separating pad 204 includes a plurality of hooks (not shown in FIG. 8) to which a spring 216 is secured so as to regulate a pressure applied by the separating pad 204 to the sheet separating roller 203, i.e. a sheet separating pressure.
Japanese Patent Publication No. Hei 5-032,296 discloses a sheet feeding apparatus, which includes a mechanism for disabling the rotation of a sheet separating/feeding roller when a sheet separating pad displaces itself from the sheet separating/feeding roller by an amount which is above a predetermined value, i.e. when two or more sheets are simultaneously separated and fed (called "multiple sheet feeding" hereinafter). In the case of multiple sheet feeding, this mechanism is intended to interrupt sheet feeding at an inlet of a sheet feed path, and to prevent the register rollers from being damaged by sheets which are simultaneously fed and have a certain thickness.
Further, Japanese Utility Model Publication No. Hei 4-023,862 describes a technique to overcome a disadvantage of a tilting sheet tray. Specifically, such a sheet tray tends to cause multiple sheet feeding when the sheet tray is lifted and becomes substantially flush with a sheet separating roller, and does not function as a front enclosure as sheets are being paid out therefrom. In this publication, the sheet tray is attached to a free end of an arm which is rotatably supported. An inclination of the sheet tray is controlled in accordance with a length of the arm.
In the prior art shown in FIG. 8, in order to regulate the sheet feeding or separating pressure, it is necessary for an operator to manually change a position for securing the spring with a hook, which is very troublesome. Further, the sheet feeding or separating pressure delicately varies with kinds of sheets (e.g. thickness and quality of paper). Thus, this adjustment requires dexterity and quick response of the operator, and is not performed so reliably.
Japanese Patent Publication No. Hei 5-032,296 relates the technique which is applied after multiple sheets are fed at a time, but it is not intended to prevent multiple sheet feeding. Thus, whenever multiple sheet feeding is caused, the printing operation should be suspended so as to remove jammed sheets.
The last mentioned publication relates to the technique for preventing multiple sheet feeding simply by paying attention to the function of the sheet tray as the front enclosure. No measures are taken into consideration from the viewpoint of the sheet feeding pressure and the kind of sheets.
In order to reliably feed each sheet to a printing section, the sheet feeding apparatus is continuously required to maintain appropriate sheet feeding and separating pressures in accordance with the kinds of sheets. However, it does not always follow that once the sheet feeding and separating pressures are appropriately determined with accordance with the kinds of sheets, neither no-sheet feeding nor multiple sheet feeding should take place.
This is because the factors affecting the sheet feeding and separating pressures, i.e. the thickness and quality of sheets, tend to vary with environmental conditions in the printer (e.g. temperature, humidity and so on).
Further, the foregoing sheet feeding apparatus is disadvantageous in the following respect: no-sheet feeding caused during use by a worn feed roller; or waste of time when the operation is suspended in order to replace a worn-out feed roller. At present, no particular measures have been taken from this viewpoint.
Even when a sheet feeding pressure and a sheet separating pressure are automatically adjustable by a sheet feeding pressure regulating mechanism and a sheet separating pressure regulating mechanism, either no-sheet feeding or multiple sheet feeding caused by such a sheet feeding or separating pressure should be attended by the operator (i.e. the operator should manipulate the control panel so as to resume the automatic adjustment). This job requires the operator's dexterity and quick response.